Which historic trade route took the longest in (time) to travel and how long did it take? Even better would be if somebody knew the commodities that were transported on the route.
The Manila Galleon might be a contender if you add the overland and transatlantic parts of the trip. Manila to Acapulco or Panama, Panama to Havana, Havana to Seville.
Do you count the round trip, if each leg waits for once-a-year seasonal winds?